Depending on your needs there can be several different approaches.
-Add a (thin) layer to the existing wall with the paint material assigned.
-Add a thin single layer wall and put it against the regular wall.
-Use sweeps with paint material assigned.
-Create a wall/face based family with the paint material and stick it to the walls were needed.
Louis
![EESignature EESignature](http://autodesk.i.lithium.com/t5/image/serverpage/image-id/1272565i464DC3DC646B4F11/image-size/original)
Please mention Revit version, especially when uploading Revit files.